1. Field of the Invention
The present invention relates to sound reproduction systems and more particularly to systems and methods for multi-dimensional sound reproduction utilizing multiple audio channels to generate multiple soundfields.
2. Description of the Related Art
Modern day stereo audio reproduction systems provide sound reproduction more realistic than monophonic systems because two (2) soundfields are utilized to give the listener an impression closer to that of a live performance which generates sound from multiple sources located at multiple points in space relative to the listener. However, to more truly reproduce live performances considerably more than two soundfields are needed.
In fact, sound which is heard by a listener arrives via multiple paths. Besides a direct or line of sight arrival, sound often is reflected or bounced off walls and objects prior to arriving at the listener's ears. The multiple paths from a sound source may be characterized as direct, reverberations, and indirect late arriving reverberations which may be measured with digital computing equipment. The multiple arrival of sound causes perceptibly phase shifted signals at the listener which tends to compress the soundfield.
Basic problems are associated with conventional stereo recording and reproduction equipment which tends to cause sound to be focussed and compressed. Since microphones pick up sound from single points in space, they tend to compress recorded sounds around a point. On the other hand, when speakers reproduce sound, they tend to highly focus sound in a single direction forward and away from the speaker. This accounts for the basic problem of limited and unnatural coverage associated with conventional stereo.
Over the past several years, numerous formats utilizing considerably more than two channels have been developed for more realistic spatial sound reproduction, sometimes referred to as three-dimensional or surround sound reproduction systems. Such systems have found practical application in home theater environments. Home theater, however, has proven to be a constantly evolving technology. Practical implementation of a home theater surround sound system has thus turned out to be problematic. A standardized home theater format has not been adopted. Quite to the contrary, each of the formats developed tend to create differing sonic impressions irrespective of the particular loudspeakers utilized.
Ambisonics traces its roots to the quadraphonic sound. Quadraphonic sound, while a commercial disaster, did have elements of superiority although blemished with sonic impressions that would migrate, blurr or smear. All compact discs produced by Nimbus Records have been Ambisonic encoded for a decade. Only one microphone is used which fully covers soloists and the ambience of an orchestra where a performance is given. This microphone consists of four recording diaphragms spaced apart at 90 degrees from one anther. Although well received in Europe, Ambisonics has been slow to gain acceptance in the United States. Ambisonics was developed by the National Research and Development Council in the United Kingdom. The general thesis was authored by Michael Gerzon. Two decoders were produced by Minims and Troy in the 1970s.
The Ambisonics format preceded Dolby Surround Sound. Dolby Labs saw Ambisonics' benefits for the cinema where Dolby noise reduction technology had become a household word. By extracting difference information from the primary stereo signals (left-right), vocals were cancelled from back speakers and subwoofers. This left only non-directional, spacial information. Vocalists are dominantly monophonic. Thus, audio channels were doubled from two to four. A fifth (sum total left plus right) channel was added to anchor the vocals to the center of large video screens. Where wide aspect screens are utilized the three sound channels--left, center, right--faithfully follow the action on the screen. Dolby Surround Sound's contribution to Home Theater was the addition of time delay to rear ambience loudspeakers. Adjustable delay (19 to 90 ms) is used to preserve the audio integrity between what viewers see on the screen at close range and surround sounds of long duration arriving from behind the audience.
The Stereo Retrieval System (SRS) was developed by Arnold Klayman originally for automotive applications. SRS strives to recreate a three dimensional surround sound effect by restoring the spatial elements lost in conventional recording technology. Studies have shown that microphones record only a small fraction of what is in the near field. SRS also records "ambience"--spatial characteristics that surround the musical event. Without ambience the sonic impression is flat and colorless--"canned". Extracting ambience, equalizing with the main stereo channels then feeding it back into the stereo channels SRS creates in greater realism. It is this realism that Arnold Klayman restores in the SRS surround sound giving a 3-D sonic impression.